The hybrid vehicle generally has two different power sources simultaneously. According to the various connection types, it may be mainly divided into three structures of series hybrid, parallel hybrid and series-parallel hybrid.
The series hybrid is characterized in electrically coupled, wherein the engine directly drives an electric generator for charging the battery, and then an electric motor is operated by electrical energy from the battery to drive the wheels. The parallel hybrid is characterized in mechanically coupled, wherein the engine is connected with the driving shaft, and the motor can be selectively operated as an electric motor or a electric generator for balancing the loads of the engine.
In the prior art, one of the series-parallel hybrid may use a planetary gear unit to realize the distribution of power among the driving units. In this structure, the engine may be connected to the carrier of the planetary gear unit, a part of the power of the engine is transferred into the electrical energy by means of the first motor for furnishing the electrical energy to the second motor or for charging the battery, and the other part of the power of the engine is directly transmitted to the ring gear of the planetary gear unit; meanwhile, the second motor is connected with the ring gear for the supply of some supplementary power or torque. In this structure, a part of the power output from the engine is always used to power the first motor to generate the electrical energy for charging the battery or for driving the second motor.
In the prior art, a series-parallel hybrid may use the clutch to realize the power distribution among the driving units. For example, the Chinese patent CN 1200824 C discloses a hybrid power drive system for propelling the vehicle, which comprises two clutches, two motors and a battery, wherein the first clutch and the second clutch are respectively disposed between the first motor, the second motor and the internal combustion engine, the second motor is permanently connected to a variable-ratio transmission, and the first motor and the second motor are connected electrically with the battery. As it can be seen from the disclosure, the first motor of the drive system has three functions as below: (1) starting motor for the internal combustion engine; (2) generator for outputting electrical energy; and (3) drive device for driving the transmission hydraulic pump and other auxiliaries such as an air conditioner compressor, a power steering pump. Thus it can be seen that, the first motor is equal to a starting motor (with lower power or capacity) for the internal combustion engine in a conventional vehicle, except that it is electrically connected with the battery, and under control, it has some other functions besides to start the internal combustion engine, for example, to drive some auxiliaries. Moreover, the second motor can be used in a purely electrical operation only when the vehicle is starting from the rest or is driving at a low speed. When a higher torque is needed, the engine must be started to propel the vehicle alone or together with the second motor. In light of the performances and functions of the first motor and the second motor as mentioned above, the whole drive system is limited in designs and functions. On the one hand, the second motor must be permanently connected with a variable ratio transmission to efficiently output the power, that is, the deficiencies of the second motor in function and output capacity must be compensated by changing the various shifts of the transmission; moreover, during the changing of shifts, the power transmission to the wheels will be interrupted, which may reduce the comfort of the vehicle. On the other hand, when the first motor fails to supply sufficient torque for starting the internal combustion engine, and the second motor is used for starting the internal combustion engine, the variable ratio transmission must be in neutral so that the second motor and the wheels must be under the no-torque transmission condition, otherwise, the second motor will also, via the transmission, drive the wheels to rotate (when the vehicle is at rest, if the transmission is not in neutral, the second motor can not start the internal combustion engine). Since the variable-ratio transmission must be used, the drive system will occupy more space on the vehicle and the cost will be increased.
As mentioned above, due to the limitation in function of the first motor, the first motor can't be operated for driving the wheels alone, and unsuitable for driving the wheels together with the second motor, thus the drive system only has some simple drive modes and functions. For various traveling conditions of the vehicle, the drive system can't provide the optimum power output for multiple-functions and multiple-modes driving, thus the power efficiency is not optimized, and the optimized power output and reduction of fuel consumption can not be realized.